net: fec_mxc: change fec_mii_setspeed parameter

Only the hardware ethernet registers are needed
for this function, so don't pass the more general
structure. I'm trying to separate MII and fec.

This also fixes MX28 fec_mii_setspeed use on secondary ethernet port

This was found by inspection of the code and should be
checked on real hardware.

Signed-off-by: Troy Kisky <troy.kisky@boundarydevices.com>